Despite Extinction Predictions, Mountain Streams Supporting Fish

Predictions that fishes living in cold mountain streams were set to suffer massive extinctions thanks to climate change have turned out to be wrong, according to new research led by scientists at the U.S. Forest Service.

Instead of withering away as thought, the fish that live in the streams, typically salmon or trout, have only seen minor temperature increases over time, researchers say. And those haven’t been anywhere near extreme enough to kill off the populations, though there still have been some negative consequences.

Results of the work help to solve a longstanding mystery that has been puzzling scientists for years: Why, with all the projected declines, have fish living in mountain streams been able to carry on without much incident?

“Predictions of widespread species losses, however, have yet to be fulfilled despite decades of climate change, suggesting that trends are much weaker than anticipated and may be too subtle for detection given the widespread use of sparse water temperature datasets or imprecise surrogates like elevation and air temperature,” scientists write in the study published in the journal Proceedings of the National Academy of Sciences.

Researchers wanted to get at the answer, but they also wanted to keep from spending too much money on a complex, field-intensive investigation. So they used existing data records instead, analyzing stream temperature measurements from more than 100 agencies and a regional climate model built by the U.S. Geological Survey to assess warming trends in 138,000 miles of streams in the U.S. Northwest. States covered in the effort included Idaho, Oregon, Washington, western Montana, as well as small portions of western Wyoming, northern Nevada, northern Utah and northern California.

Northwest United States temperature and climate map developed from data at more than 16,000 sites that was used to highlight climate refugia for mountain stream species. (Credit: Dan Isaak / U.S. Forest Service)

The scientists found that stream temperatures have warmed at an average rate of 0.10 degrees Celsius (0.18 degrees Fahrenheit) per decade over the last 40 years. This translates to thermal habitats shifting upstream at a rate of only 300 to 500 meters (0.18 to 0.31 miles) per decade in headwater mountain streams where many sensitive cold-water species currently live. These observed changes underscore that climate change is still detrimentally affecting the habitats of those species, but at a much slower rate than dozens of previous studies have put forth.

The results mean that many populations of cold-water species will continue to persist in this century and that mountain landscapes will play an increasingly important role in preserving them.

The findings are unexpected, as scientists note that the cold headwater streams that were believed to be most vulnerable to climate change actually appear to be some of the least vulnerable. And the fact that they were gleaned used existing data is likewise surprising because the measurements had been there all along.

Scientists say that their results mean resource managers will have more time to complete extensive biological surveys of ecological communities in mountain streams than they had planned for. The extra time will allow for conservation planning strategies that can adequately address all species.

The findings of the work build on others made in the Cold-Water Climate Shield effort, which looked to combine data and other information to provide a foundation for planning conservation networks to preserve native trout populations in the northwestern U.S.

Funding for the study was provided by the U.S. Fish and Wildlife Service Great Northern and North Pacific Landscape Conservation Cooperatives.

Top image: Bull trout, a popular fish species of conservation concern, that finds shelter in mountain streams. (Credit: Bart Gamett / U.S. Forest Service)

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